The effect of space travel on human reproductive health: a systematic review

With increasing possibilities of multi-year missions in deep space, colonizing other planets, and space tourism, it is important to investigate the effects of space travel on human reproduction. This study aimed to systematically review and summarize the results of available literature on space travel, microgravity, and space radiation, or Earth-based spaceflight analogues impact on female and male reproductive functions in humans. This systematic review was performed according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines and Space Biomedicine Systematic Review methods. The search was performed using three databases: PubMed, Web of Science, and Medline Complete. During the database search, 364 studies were identified. After the study selection process, 16 studies were included in the review. Five studies included female participants, and the findings show an increased risk of thromboembolism in combined oral contraceptive users, decreased decidualization, functional insufficiency of corpus luteum, and decreased progesterone and LH levels related to space travel or its simulation. Male participants were included in 13 studies. In males, reproductive health considerations focused on the decrease in testosterone and sex hormone-binding globulin levels, the ratio of male offspring, sperm motility, sperm vitality, and the increase in sperm DNA fragmentation related to space travel or its simulation. Results of this systematic review highlight the need to focus more on the astronaut’s reproductive health in future research, as only 16 studies were found during the literature search, and many more research questions related to reproductive health in astronauts still need to be answered.


INTRODUCTION
To undertake multi-year missions in deep space, colonize other planets, and/or prepare appropriate safety measures for space tourism, it is important to investigate the possible effects of space travel on human reproduction.During space travel, astronauts are exposed to several hazardous factors, such as alterations in gravitation forces, including hypogravity and hypergravity, or ionizing radiation 1,2 .Exposure to microgravity has been demonstrated to impair the endocrine system in males 3 , muscle mass, and bone mass 4,5 ; it also leads to altered fluid and electrolyte balance, cardiovascular changes, or increased glomerular filtration rate in both genders 2 .Experimental bed rest studies are typically used in humans to simulate spaceflight microgravity 6 .For in vitro samples, clinostat or random positioning machines are used to simulate microgravity by randomization of the gravity direction over time.In animal studies, hindlimb suspension is usually used to stimulate the physiological effects of microgravity 2 .Ionizing radiation, which is about 500 times greater in space compared to Earth conditions, was observed to cause DNA damage, apoptosis in ovarian follicles, and sperm DNA fragmentation in animal models 1,2,7,8 .
Until now, limited research has focused on the effect of space travel on the reproductive system and its function, along with endocrine regulation of reproduction or prenatal development.Endocrine regulation of sex hormones is the most investigated as it also impacts musculoskeletal health and skeletal muscle protein metabolism (e.g. 9,10).Most of this research is based on animal models 1,2 .
Female mouse models show that microgravity affects embryonic stem cell growth and differentiation 11 , resulting in impaired decidualization of the endometrium needed for implantation and maintaining pregnancy 12 .Data from therapeutic radiation on ovaries suggests that space radiation exposure during a typical Mars mission may reduce the ovarian reserve by 50% by destroying some of the primordial follicles.Furthermore, space radiation may lead to a decreased time interval to menopause, leading to a decreased reproductive capacity of the female astronaut 13 .Exposure to total body radiation of 15 Gy leads to the loss of ovarian function in humans 14 .
In males, the microgravity exposure was observed to reduce the total sperm count in mice models 15 , decrease testes weight, and decrease testosterone concentrations in male rats 2,16 .Exposure to ionizing radiation increases sperm DNA fragmentation in Echinogammarus marinus models 7 , affecting the male reproductive health.Data from therapeutic radiation on testes in humans show that a dose higher than 1 Gy might result in azoospermia and risk for hereditary disorders 17 .Furthermore, decreased serum testosterone levels were observed in men treated with radiotherapy for rectal cancer when testes are exposed to direct or scattered radiation 18 .
As described in The Impact of Sex and Gender on Adaptation to Space: A NASA Decadal Review 19 , reproductive demographics of female and male US astronauts significantly differ based on biological processes and gender roles for parents.Women are usually the primary caregivers (e.g., ref. 20 ) and are often required to take an extended family leave from their career when having a child (e.g., ref. 21).A smaller number of female astronauts (44.7%) have at least one child compared to the male astronauts (83.9%).Female astronauts also significantly delay reproduction, on average, by 5.6 years compared to males.It was hypothesized that the delayed reproduction in female astronauts is related to the required extensive space travel training time 19 .A new NASA decadal review is expected next year, adding more current data on the topic.However, not only delayed reproduction but also the impact of the potential acceleration of aging and gonadal radiation exposure related to space travel might be other factors affecting the reproduction capacity in female astronauts 19 .Therefore, the aim of this study was to systematically review and summarize the results of available literature on space travel, microgravity, and space radiation's impact on female and male reproductive functions in humans.

Characteristics of included studies
During the database search, 364 studies were identified (Pubmed: 121 articles; Medline Complete: 142 articles; Web of Science: 101 articles).After the duplicate removal (n = 160), studies involving animal samples (n = 45), different methodologies, or non-English language articles (e.g., conference paper, book chapter; n = 12), 147 studies were screened based on title and abstract in which 120 studies were excluded.In the last stage, 27 studies remained, followed by the exclusion of 5 studies due to no full-text being available and eight studies because the outcomes needed to match the topic.Via another method (accidental find), two studies focusing on ionizing radiation's effect on male reproductive health were found 22,23 .A total of 16 studies were included in the systematic review.
The methodological quality of the included studies (Table 1) ranged from 70.0% 24 to 90.9% 25 , suggesting good methodological quality 26 .The most common methodological deficits consisted of not reporting the study's hypothesis clearly, not reporting the probability values, and the lack of representativeness of the source population.The methodological quality of the included studies is shown in Supplementary Table 1.
Thirteen articles reported data from the experiments on Earth.Simulated microgravity by parabolic flight was used in two studies 31,35 and by clinostat system in one study 12 ; in two studies ionizing radiation occupational exposure was used 22,23 , headdown bed rest was performed in four studies 10,27,28,30 , bed rest was conducted in two studies 6,29 , in two studies dry immersion was used 24,32 , and in one study data from water submersion were reported 25 .Three articles reported data on space exposure 30,33,34 .Eight of included studies focused on endocrine changes after the space flight or its simulation 10,25,6,27,28,29,30 ; four studies focused on sperms 22,23,31,35 ; two studies focused on menstrual cycle  Strollo et al. 27 Smith et al. 30 Belavy et al. 6 Liang et al. 28 Smorawinski et al. 29  changes 24,32 ; one study focused on endometrial stromal cells 12 , one study focus of the venous thromboembolism risk 34 , and one study focused on offspring sex ratio in male astronauts 33 .

Female and mixed studies
Three studies, including female participants, and two studies, including mixed samples, were identified during the study selection process.Their characteristics are shown in Table 4.One study used a clinostat system to simulate the microgravity of in vitro samples of human endometrial stromal cells (PSCs) from the uterus 12 .Two studies used dry immersion to simulate microgravity; in both studies, participants were allowed 30 min/day to spend outside the immersion bath for hygiene procedures 24,32 .Tomilovskaya et al. 32 described the first female dry immersion study, and their participants were involved in their study for two menstrual cycles.The three-day dry immersion occurred between day 7 and day 10 of participants' menstrual cycles 32 .In a study by Gorbacheva et al. 24 , two menstrual cycles were followed; the 5-day-long dry immersion was performed between day 10 and day 15 of participants' menstrual cycles.One study used one hourlong -12°head-down bed rest to simulate microgravity and analyze endocrine parameter changes 27 .In one study, spaceflight exposure was used to analyze proteins involved in clotting cascade from blood samples obtained twice before spaceflight, five times during the flight, and twice after flight 34 .
In Table 5, results from female and mixed samples studies are shown.In female in vitro samples, exposure to microgravity was observed to decrease decidualization (the process of endometrial cells in preparation for, or during pregnancy) by decreasing proliferation and migration and endometrial stromal cells growth rate through Akt/MMP and FOXO3a/autophagic flux 12 .Two studies reported that menstrual cycle length stayed intact after dry immersion exposure 24,32 .Gorbacheva et al. 24 observed decreased luteinizing hormone (LH), progesterone, and ovarian volume at day 9 of the menstrual cycle after the immersion.On the other hand, an increase in dominant follicle diameter and no change in uterus size and endometrial thickness were reported 24 .
The mixed sample study focused on venous thromboembolism risk in male and female astronauts, showing an increased risk in females taking combined oral contraceptives 34 .A head-down bed rest mixed sample study reported no statistically significant change in oestradiol, testosterone, and LH levels after the rest 27 .

Male studies
Eleven studies focused on male participants were identified during the study selection process.In Table 6, the male studies' characteristics are summarized.In the study by Little et al. 33 ,  retrospective data from astronauts' biographies were included in the analysis of offspring ratio.A study by Smith et al. 30 focused on the effect of long-and short-duration space flight and −6°headdown bed rest on testosterone levels.In two studies, parabolic flights were used to simulate short-duration microgravity 31,35 .In a study by Boada et al. 35 , twenty parabolic flight maneuvers (8.5 s of microgravity for each parabola) were used for frozen sperm samples.In a study by Ikeuchi et al. 31 , fresh sperm samples underwent ten parabolic flight maneuvers (20 to 25 s of microgravity for each parabola).Occupational low-dose exposure to ionizing radiation while working with radiation in a hospital on sperm characteristics was analyzed in two studies 22,23 .Four studies analyzed the effect of bed rest or −6°head-down bed rest on testosterone levels 10,6,28,29 .Studies by Belavy et al. 6 , Liang et al. 28 , and Zachwieja et al. 10 applied a strict bed rest.Study design by Smorawinski et al. 29 provided 20 min/day to ambulate (to shower and toilet).In one study, the effect of water submersion on testosterone levels was analyzed 25 .In the study by Loder et al. 25 , divers were allowed to emerge for less than 20 min every 4 hours to urinate, defecate, drink, or undergo medical checks.
One of the studies including male participants in the space study setting (Table 7) focused on the offspring sex ratio, showing a decreased ratio of male offspring (43.75%) in male astronauts 33 .Furthermore, the study by Little et al. 33 observed a decreased male offspring ratio of 38.41% in high G pilots compared to 50.34% in low G pilots.The second study using the space study setting focused on endocrine changes, showing no statistically significant changes in testosterone and sex hormone-binding globulin (SHBG) during or after the short and long-duration space flight.A decrease in total, free, and bioavailable testosterone was observed only on the landing day after the space flight, probably as the transient effect of flight 30 .One study observed decreased sperm motility after microgravity exposure 31 , and another study by Boada et al. 35 observed no statistically significant change in sperm motility, vitality, or sperm DNA fragmentation after exposure to microgravity.Occupational ionizing radiation exposure was observed to decrease sperm motility, vitality, and concentration and to increase sperm DNA fragmentation in comparison with non-exposed controls 22,23 .Bed rest and headdown bed rest studies show no statistically significant change in testosterone and prolactin after the rest 10,28,29,30 .SHBG was observed to decrease after the bed rest 6 .After the water submersion, a decrease in plasma testosterone was observed 25 .

DISCUSSION
The aim of this study was to systematically review and summarize the results of available literature on space travel, microgravity, and space radiation impact on female and male reproductive functions in humans.The reproductive health consideration of space travel differs for female and male astronauts.In female astronauts, they include oral contraceptive use 34 , progesterone and LH levels 27 , ovarian and uterus changes 24 , decidualization, and endometrial stromal cell growth rate 12 .In males, reproductive health considerations focus on testosterone and SHBG levels 10,25,6,27,28,29,30 , the ratio of male offspring 33 , sperm motility 22,23,31,35 , sperm vitality 22,35 , and sperm DNA fragmentation 22,23,35 .To support those considering these options, it might be helpful to explore assisted reproductive technologies such as oocyte and sperm cryopreservation, along with reproductive counseling possibilities, as suggested by Rose 13 and Ronca et al. 36 .
In female astronauts, the endocrine regulation of the menstrual cycle involves the hypothalamic release of gonadotropin-releasing hormone, which stimulates the pituitary gland to produce folliclestimulating hormone and luteinizing hormone, which peaks midcycle and invokes ovulation 37 .The developing ovum in ovaries produces estrogen, and the corpus luteum, which forms after ovulation, produces progesterone.Animal models show a decrease in luteinizing hormone related to 37 days-long spaceflights; however, no changes in estrous cycle stages were observed 38 .In naturally cycling women, simulated microgravity by dry immersion led to a decrease in luteinizing hormone by 12% and progesterone by 52%, showing functional insufficiency of corpus luteum 24 .The menstrual cycle length was not altered after 3 and 5 days of dry immersion 24,32 .One hour of −12°head-down bed rest did not induce any significant changes in the endocrine regulation of the cycle, suggesting that longer microgravity exposure is needed to affect the endocrine regulation of the menstrual cycle 27 .arth ≈ Smorawinski et al. 29 Earth ≈ Zachwieja et al. 10 Earth ≈ Loder et al. 25 Earth ↓ ↓, statistically significant decrease; ≈, no statistically significant difference; ↑, statistically significant increase; BR bed rest, SHBG sex hormone-binding globulin.
As the menstrual bleeding flow management during flight training and the space flight can be challenging, medically induced amenorrhea using combined oral contraceptives is often used by female astronauts 40 .However, combined (progestin and estrogen) oral contraceptives were associated with lower circulating concentrations of albumin, higher concentrations of transferrin, and elevated markers of inflammation, which can contribute to an increased risk of venous thromboembolism event during space travel 34 .The occlusive deep venous thrombosis was diagnosed in one female astronaut during a long-duration spaceflight 41 , highlighting the need to carefully consider the type of combined oral contraceptives used before and during flight 34 .
Human pregnancy is currently contradicted during space flight as a safety measure to protect the fetus 13,42,43 .Multi-year duration space flights and colonization will require understanding the impact of space flight on pregnancy, and simulation studies will try to provide better insight into reproduction in space.Fetal development, long-term effects on gestation under space conditions, and monitoring the development and function of offspring conceived and developed in space are some of the potential priorities for future space programs as described in a European perspective of human development and reproduction in space by Jain et al. 43 .The study by Cho et al. 12 showed that exposure to simulated microgravity leads to decreased decidualization and endometrial stromal cells growth rate due to decrease in Akt activity and FOX03a expression leading to an unreceptive endometrium.Furthermore, if microgravity and space radiation alter the pro-oxidant/antioxidant balance during pregnancy, it can increase the risk of miscarriage, preterm birth, or fetal growth restriction 44 .The absence of gravitational loading during the last trimester of gestation may cause hypotrophy of muscles and osteopenia in the trunk and legs, leading to delayed acquisition of developmental milestones such as sitting or walking of the fetus developed in space 45 .Animal models show increased perinatal morbidity for the rats that spent 9 to 20 days in spaceflight during their gestation.In surviving offspring, no delay in walking acquisition was observed 46 .
High-altitude airplane flights, e.g., transatlantic flights, constitute trivial cosmic radiation exposure for casual travelers.Pregnant pilots, flight attendants, and frequent flyers may exceed the recommended radiation exposure 47 .During transatlantic air travel in the third trimester of pregnancy, most of the pregnant women report no change in fetal movements during take-off or flight 48 .A study by Grajewski et al. 49 focusing on miscarriage risk among flight attendants shows that cosmic radiation exposure of 0,1 mGy or more may be associated with an increased risk of miscarriage in weeks 9 to 13.However, the miscarriage risk was also associated with other factors such as work during sleep hours and high physical demands, and the miscarriage risk was not increased among flight attendants compared to a control group of teachers 49 .Maternal stress and exposure to stressful events during pregnancy were observed to impact the infant's physical health 50 , premature birth, and low birth weight 51 , suggesting a possible negative effect of space travel-related stress on the fetus.
Space travel may increase the carcinogenic risk to reproductive organs.This risk was proposed to be higher in women as they have a higher incidence of radiation-induced cancers, as widely discussed in Market al. 19.Still, the low number of female astronauts does not allow for assessment of the risk of spaceflight on gynecological cancer 36 .
In expert opinion by Rose 13 , significantly reduced ovarian reserve and consequent decrease in the reproductive capacity and decreased time interval to menopause caused by space radiation was suggested in female astronauts.Unfortunately, no original article showing the data about reproductive capacity or age of menopause in astronauts was found during the literature search in this systematic review.
Testosterone is the key hormone in the development of the male reproductive system and promotes muscle and bone mass 52 .Testosterone has been, therefore, often considered as a potential countermeasure for musculoskeletal losses related to space flight (e.g. 10 ).The testosterone level seems unchanged by the space flight or bed rest study settings 9,10,6,27,28,30 apart from the transient effects after flight 30 .A decrease in testosterone levels was observed in a short-term water submersion (41 h) study by Loder et al. 25 .Similarly, it was hypothesized that the decrease is related to stress effect 25 .The self-rated sexual drive was reported to temporarily decrease during space flight in male astronauts parallelly to urinary, plasma, and salivary testosterone levels in a study by Strollo et al. 53 .Similarly, animal studies show a decrease in testosterone levels in simulated microgravity studies caused by a reduction in testicular blood flow related to body fluid shift 1 .
Prolactin and LH levels did not change during the analog bed rest study 27,6 .Similarly, no LH and FSH levels change was observed after a 6-week hindlimb suspension in animal models 54 .Serum SHBG levels were observed to decrease during bed rest in inactive participants.The physical activity load during the bed rest led to stable SHBG levels 6 .Similarly, no change during or after the space flight in the level of SHBG was observed by Smith et al. 30 Results observed by Ikeuchi et al. 31 using fresh semen suggest that sperm motility is reduced by microgravity.In a study by Boada et al. 35 using frozen semen, no significant change in sperm motility, vitality, or sperm DNA fragmentation was observed compared to Earth condition after a similar parabolic flight experiment as used by Ikeuchi et al. 31 .These results suggest that the sperm integrity may be protected by cryopreservation during the space flight when transporting male human gametes into space 35 .Still, chronic occupational exposure to ionizing radiation was observed to have a detrimental effect on sperm motility, vitality, concentration, and DNA fragmentation 22,23 .Similarly, ionizing radiation and microgravity were observed to increase sperm DNA fragmentation in animal studies 1 .Furthermore, a decreased sex ratio of male offspring by male astronauts exposed to high G forces was reported by Little et al. 33 .The authors hypothesized that sperm sex differences in sperm motility and longevity, smaller size, and cytoplasm content in Y sperm were the reason of decreased sex ratio of male offspring as higher G forces may accelerate metabolism in sperm subtracting energy available for travel to the ovum 33 .However, current knowledge shows no morphological differences between X and Y sperms in humans 55 .Still, X and Y sperms differences in genetic content may lead to differences in their stress response 56 .The study by You et al. 57 reported that the viability of human Y spermatozoa was lower after exposure to stress (e.g., different temperatures and culture periods) compared to X spermatozoa, which may result in a shift of the offspring sex ratio as observed by Little et al. 33 .Similarly, low male sex offspring ratio associated with occupational testicular radiation exposure was observed in a previous study 58 .On the other hand, no association between offspring sex ratio and gonadal irradiation was observed in childhood cancer survivors in a study by Reulen et al. 59 Future studies on the effect of space radiation on both fresh and frozen semen samples are needed to assess the possibility of creating a human sperm bank outside the Earth.A study by Wakayama et al. 60 analyzed the effect of space radiation on mouse freeze-dried spermatozoa stored for almost six years on the International Space Station.The sperm DNA and fertility were not affected after the storage outside the Earth compared to control preserved on Earth, and the current data show the possibility of storing freeze-dried spermatozoa for more than 200 years in space 60 .
Among potential priorities identified by Jain et al. 43 for future research regarding reproductive aspects of space flight were topics similar to those covered in this systematic review.Additionally, the effect of space travel on libido and the possibility of pregnancy and birth in space were proposed 43 .Results of this systematic review highlight the need to focus more on both female and male astronauts' reproductive health in future research, as only 16 studies were found during the literature search, and many more research questions related to reproductive health in female and male still need to be answered.
There are several limitations of this systematic review.The main limitation is the few included studies and the wide range of reproductive health parameters they focused on.The small sample sizes, different types of populations (healthy volunteers, astronauts), and different methodologies need to be considered when comparing or generalizing the results.The limited number of studies addressing these health concerns underscores the imperative need for future research dedicated to reproductive health in both female and male astronauts.

METHODS Eligibility criteria for selecting studies
A systematic review of the effect of space travel or its simulation, e.g., bed rest studies, microgravity simulation, or dry immersion, on reproductive health in human females and males was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines 61 and according the Space Biomedicine Systematic Review methods (https://sites.google.com/view/sr-methods/home).The search was performed using three databases, PubMed, Web of Science, and Medline Complete, on the 29 th of April 2023 by one researcher (MG).The eligibility criteria included: (1) astronauts, space travel, or space simulation; (2) experimental or retrospective studies performed on human participants (animal studies were excluded); (3) description of reproductive health parameters.

Search strategy and selection process
The following terms with Boolean operators were used for the search: ("infertility" OR "birth outcomes" OR "amenorrhea" OR "menstrual" OR "menstrual cycle" OR "follicular phase" OR "luteal phase" OR "menstruation" OR "ovarian cycle" OR "ovulation" OR "anovulation" OR "reproduct*" OR "obstetric*" OR "gynecolog*" OR "maternal" OR "pregnan*" OR "contracept*" OR "prenatal" OR "postpartum" OR "preconception" OR "women's health" OR "miscarriage" OR "pregnancy loss" OR "menarche" OR "polycystic ovary syndrome" OR "menopause" OR "endometriosis" OR "stillbirth" OR "placental abruption" OR "low birth weight" OR "preterm birth" OR "in vitro fertilization" OR "irregular periods" OR "sperm" OR "testosterone" OR "semen quality" OR "oligospermia" OR "semen" OR "testis" OR "testes" OR "testicular" OR "offspring" OR "reproductive hormone" OR "asthenozoospermia" OR "oligozoospermia" OR "oligoasthenozoospermia" OR "oligoasthenoteratozoospermia" OR "teratozoospermia" OR "spermatogenesis" OR "varicocele" OR "erection" OR "libido" OR "erectile dysfunction" OR "sexual drive") AND ("space travel" OR "astronaut*" OR "spaceflight" OR "space analogue" OR "cosmonaut*" OR "space simulation" OR "zero gravity" OR "microgravity" OR "hypogravity" OR "low gravity" OR "space radiation") AND ("human" OR "participant*" OR "women" OR "men" OR "woman" OR "man") NOT ("review").The literature search did not exclude any studies published before certain data due to a limited number of scientific studies focused on the analyzed topic as proposed in Ahrari et al. 1 .Studies published until April 2023 were included in this study.Exclusion criteria included animal studies, non-English language, review articles, conference papers, books, and book chapters, and no full-text available.All studies identified in the search were imported into Rayyan systematic review software 62 to continue the selection process.Studies that did not meet the inclusion criteria (e.g., duplicates, non-English articles, reviews, conference papers, books and book chapters, and animal studies) were excluded by one researcher (MG).The title and abstract of the remaining studies were screened by two researchers (MG, ACP).Any disagreement between researchers was resolved by discussion.After that, the full texts of the included studies were screened to confirm their relevance to the current systematic review.The PRISMA flow diagram summarizes the study selection process (Fig. 1).

Table 1 .
Final score of methodological quality of included studies.

Table 3 .
Overview of measured outcomes of the 16 included studies.

Table 4 .
Female and mixed studies participants' characteristics and study settings.
* 8 users of combined oral contraceptives, 5 non-users of combined oral contraceptives; rpm, rotations per minute.

Table 5 .
Female and mixed studies' results.

Table 6 .
Male studies participants' characteristics and study settings.